Thermal cure monitoring of phenolic resole resins based on in situ fluorescence technique

The thermal curing reaction of two phenolic resole resins is monitored using the fluorescence technique. The intrinsic fluorescence can be used as an indicator for cure monitoring for the first resole. As the thermal curing proceeds, the intrinsic fluorescence intensity of the resole resin decreases and exhibits a few nanometers of redshift. The fluorescence intensity of the emission maxima is correlated with the conversion measured by differential scanning spectroscopy. A linear correlation is found at three different temperatures. The intrinsic fluorescence cannot always be used for monitoring the curing process of phenolic resole resins. Thus, three intramolecular charge transfer compounds and two organic donor–π‐acceptor salts are selected and applied for the cure monitoring of the second phenolic resole resin. As the curing reaction proceeds, the fluorescence emission spectra of the probes exhibit a blue spectral shift and the intensity changes because of environmental changes. An intensity ratio method is applied in which the ratios of the low‐ to high‐intensity changes in the emission bands are used to determine the degree of the curing process. There is a smooth correlation between the intensity ratio method and the degree of cure. The method enables one to follow the changes in the polymer structure at low and intermediate degrees of the curing process (below 70%) and obtain comparable results from different types of probes during the same curing process. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1773–1780, 2002     

Shelf life of phenolic resole resins

Viscosity, number-average and weight-average molecular weights (measured by gel permeation chromatography), and carbon-13 nuclear magnetic resonance (NMR) were used to monitor the changes in phenolic resoles as a function of time and pH. It was observed that phenolic resoles with pH around 5 are more stable, as indicated by the lowest increase in viscosity and molecular weight. Carbon-13 NMR spectroscopic studies also suggest that the concentration of methylol groups is higher for resoles with pH around 5 and, thereby, indicate a slower reaction.     

Structural analysis of phenolic resole resins

The chemical structure and cure characteristics of a group of phenolic resole resins were studied by means of three major analytical techniques. In particular, the effects on structure and reactivity of formaldehyde/phenol ratio and the type of reaction catalyst used were studied. Gel permeation chromatography was used to determine resin molecular weight distributions, and NMR, to determine chemical structural features. In this connection a selective oxidation procedure, converting free methylol groups to adehydes, has allowed unambiguous determination of methylene ether bridge structures to be made from the NMR data. The F/P ratio in a resole largely determines the type of molecular structures which are formed. However, triethylamine as a catalysts tends to favor methylene ether bridge formation, whereas sodium hydroxide favors methylene bridges. The rate and direction of subsequent thermal cure of the resoles prepared is shown by differential scanning calorimetry to depend markedly on the type of catalyst present during the curing stage. The DSC curing curves are interpreted in the light of the structural information provided by NMR.     

Gel permeation chromatographic analyses of resole phenolic resins

Tetrahydrofuran solutions of resole polymers were analyzed by gel permeation chromatography (GPC) on crosslinked polystyrene gel packings. The best separation was obtained with low solvent flow rates in low porosity columns. Irregular elution volumes were observed, but the effects of this erratic behavior can be eliminated by referencing retention times to that of a marker compound such as benzene or phenol. A calibration and data analysis method are presented which utilizes hydrodynamic volumes. Phenolic polymers vary in shape and ability to form hydrogen bonds with solvent; hence their molecular weights cannot be estimated from GPC data. Separation of the constituent species of resole samples is shown to be incomplete, because of aggregation between the various phenol derivatives. Particular peaks in the GPC chromatogram could generally not be assigned to individual species. Despite these limitations, GPC is a useful tool for charcterizing phenolics, and several applications are reviewed here.     

Synthesis and characterization of phenolic resole resins for composite applications

In this study, phenol–formaldehyde resins catalyzed with sodium hydroxide, triethylamine, and ammonium hydroxide were investigated and characterized in terms of their degradation behavior, flammability, mechanical properties, and chemical structure. All three resins displayed similar degradative mechanisms, and their degradation behavior was broken down into three distinct stages. These stages were attributed to the evolution of water, the volatilization of species loosely bound to the phenolic backbone, and bulk degradation of the phenolic matrix. Flammability studies were also performed on glass fiber laminates manufactured from these resins. The sodium-hydroxide- and ammonium-hydroxide-catalyzed resins were found relatively inflammable, while the triethylamine-catalyzed laminates burned readily. The mechanical properties of the resins were found to be similar or higher than the mechanical properties of other untoughened epoxies or thermosetting resins. The various chemical properties responsible for these behaviors are discussed and analyzed in terms of the catalyst basicity, solubility, and boiling point. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 505–514, 1999     

Use of sustainable glucose and furfural in the synthesis of formaldehyde-free phenolic resole resins

Sustainable resol-type resins [phenol–furfural–glucose (PFuG), phenol–furfural (PFu), and phenol–glucose resins] were synthesized via alkali catalysis. The chemical structures of the PFuG resins, which had different molar ratios of glucose to furfural, were analyzed by ¹H-NMR and Fourier transform infrared spectroscopy. A possible mechanism for formation of the PFuG resin was proposed. The crosslinking curing behaviors of the PFuG resins were examined by differential scanning calorimetry analysis. The performance of the PFuG resins as wood adhesives was studied. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47732.     

Evaluation of cure state of vinylester resins

The objective of this investigation is to characterize the effect of time and moisture on the degree of cure of two room temperature (RT)‐curable vinylester resins (VE), VE8084, and VE510A. Both resins were cured at RT for 24 h. Following the initial cure, some specimens were cured at RT over an extended period of time; others were immersed in seawater or a humidity chamber (85% RH) at 50°C, or post‐cured (PC) at elevated temperatures. Dynamic‐mechanic analysis (DMA) and differential scanning calorimetry techniques were performed to characterize the degree of cure. The degree of cure for 24 h RT‐cured and seawater exposed specimens ranged from 83 to 86% for VE510A and from 86 to 93% for VE8084. After PC at elevated temperature, or exposure to 85% RH at 50°C, the degree of cure of both resins increased to about 99–100%. Analysis of the viscoelastic response of the resins by DMA revealed two heterogeneous transitions for partially cured VE resins and a single transition for fully cured resins. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of post-curing on the mechanical properties of carbonized phenolic resins

After curing, phenol-formaldehyde resins were post-cured at 160°C, 230°C, and 300°C in air for several hours, and then those post-cured samples were carbonized at 1000°C. The effect of post-curing on the physical properties and microstructure of the carbonized phenolic resin is reported in this article. The purpose of post-curing was to improve the mechanical properties of the carbonized resins. The post-curing process promoted the crosslinking reaction and the evolution of gases. The cured resin post-cured at a higher post-curing temperature (300°C) had a significantly higher weight loss, greater linear shrinkage and lower density than others samples. During carbonization the post-curing process not only decreased the weight loss but also limited the shrinkage. Post-curing also promoted the formation of carbon basal planes and the chemical densification in structures of the final carbonized resins. The increase in post-curing temperature and time had the effect of reducing the linear shrinkage of the resin during carbonization. The TGA thermal analysis showed that the post-cured resins improved the total weight loss more than 15 wt% over the unpost-cured resin. The carbonized resins developed from the post-cured resins had a greater flexural modulus by about 10–50% and improved the linear shrinkage by about 10% over that developed from unpost-cured resin.     

Novel phenolic resins with improved mechanical and toughness properties

Novel phenolic type of thermoset resins were synthesized, and their mechanical and toughness properties were evaluated. Phenol Formaldehyde (PF) phenolic resins were modified to broaden their applications for modern composite structures. A first modification consisted of copolymerization of Phenol with Cardanol during the synthesis of resole phenolic (CPF) resins. The modified phenolic resins (CPF) were prepared at various molar ratios of total Phenol to Formaldehyde (F : P ratio) and with different weight ratios of Phenol to Cardanol. CPF resins with a maximum content of 40 wt % of Cardanol were synthesized and used. The CPF resins were applied as a plasticizer and toughening agent to the base PF resins. Both resins (CPF/PF) were mixed in different proportions, and their thermal and mechanical properties were then established. A full miscibility of the two resins was observed with the formation of a single‐phase system. An increase in the content of Cardanol resulted in a proportional increase of the flexural strength and fracture toughness together with a decrease of the flexural modulus of the cured CPF/PF resins. Further increased plasticizing and toughening effect was also observed by the blending of the CPF resins with propylene glycol. The higher toughness and flexibility effect of the CPF resins was obtained with a F : P molar ratio equal to 1.25 and with a Cardanol content of 40% (w/w). © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Nonisothermal curing of a solid resole phenolic resin

A commercial solid resole phenolic resin was thoroughly characterized with Fourier transform infrared spectroscopy, NMR, and gel permeation chromatography, and its nonisothermal curing reaction was studied systematically with differential scanning calorimetry at a series of heating rates (βs) of 3, 4.5, 5.7, and 10°C/min. The results show that the solid resole had a higher molecular weight than conventional liquid resoles, and its reactive hydroxymethyl (CH₂ OH) and dibenzyl ether (CH₂ O CH₂) functionalities participated in the crosslinking reaction upon heating. The nonisothermal curing reaction of the solid resole exhibited a relatively constant reaction heat, whereas the onset, peak, and end curing temperatures increased gradually with increasing βs. In addition, the reaction kinetics of the solid resole was analyzed with an nth‐order reaction model, the global activation energy was determined with the Kissinger method, and the reaction order was derived from the Crane equation. The obtained rate equation was applied to simulate the reaction time, conversion, and reaction rate, with a good fit achieved between the experimental data and the model predications. In conclusion, this study provided us with new knowledge on solid resoles at a molecular level and was also a great help for the curing procedure design, property optimization, and practical application of this commercial solid resole. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Interdependence between the curing,structure, and the mechanical properties of phenolic resins

The interdependence between the curing conditions, structure, and the mechanical properties of tow neat phenolic resin systems was investigated. Changes of the distribution of the void diameters were characterized by light‐ an scanning electron microscope analyses. Tensile tests and dynamic mechanical thermo analysis were performed to determine the influence of the hardener concentration and the curing temperature on the mechanical and the thermomechanical properties. The study reveals that the hardener concentration predominately influenced the microscopic structure, and thus the mechanical properties of the phenolic resin systems. By varying the postcuring times, it can be shown that independent from the microstructure of the phenolic resin system, the degree of cure has a strong influence on the mechanical properties. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3173–3185, 1999     

催化剂对Resole酚醛树脂结构与性能的影响

分别以氨水、三乙胺、氢氧化钠为催化剂合成了3种Resole酚醛树脂,通过pH值,凝胶色谱、红外光谱分析等方法,讨论了不同催化剂的催化反应机理,催化剂对Resole树脂的分子质量及分布以及树脂结构的影响。将3种树脂通过浸渍和热压工艺制备了酚醛树脂层压板,测试了其弯曲性能。结果表明,3种催化剂的催化效率依次为是NaOH>三乙胺>氨水。采用3种催化剂所合成的树脂具有相似的化学结构。采用三乙胺为催化剂时,所制备的层压板具有最佳的工艺性和力学性能。     

Master curve and time–temperature–transformation cure diagram of lignin–phenolic and phenolic resol resins

The aim of this work is to generate both a master curve of resol resins based on the time–temperature superposition principle and their TTT cure diagrams. The samples used for this purpose were lignin–phenolic and phenol–formaldehyde resol resins. A TMA technique was employed to study the gelation of resol resins. In addition, a DSC technique was employed to determine the kinetic parameters through the Ozawa method, which allowed us to obtain isoconversional curves from the data fit to the Arrhenius expression. Establishing the relationship between the glass‐transition temperature and curing degree allowed the determination of the vitrification lines of the resol resins. Thus, using the experimental data obtained by TMA and DSC, we generated a TTT cure diagram for each of resins studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3362–3369, 2007     

A dynamic mechanical study of the curing reaction of two epoxy resins

The curing behavior of two commercially formulated epoxy resins composed of the tetrafunctional amine dicyandiamide and with differing epoxy components, 4,4′-bisglycidylphenyl-2,2′-propane and the tetraglycidyl ether of methylene dianiline, is characterized by dynamic spring analysis. This supported viscoelastic technique is well suited to the determination of the onset of gelation under isothermal conditions but the method is not useful for monitoring later stages of reaction when the resins become more rigid. The activation energy for the curing of the two resins is about 87 kJ/mole (20.7 kcal/mole). Rate constants for the first order curing reaction are given. Additional studies of films cured below the ultimate T_g show that two relaxations can be observed upon heating. The first relaxation occurs near the original isothermal cure temperature with a low activation energy, about 250 kJ/mole, whereas the second relaxation occurs near the ultimate T_g, under the conditions used here, with an activation energy of 500–650 kJ/mole. It is believed that these activation energies provide a unique method of characterizing the molecular mobility of epoxy resins at various states of cure.     

Thermomechanical properties and morphology of polyethylene oxide and phenolic resole blends

Blends of poly(ethylene oxide) (PEO) and resole type phenolic resin were prepared by a solution cast method using water as a solvent. The cured blends were made by heat curing without using any catalyst. The blends were characterized by dynamic mechanical analysis (DMA), which indicated that PEO forms compatible blend with the resole. The glass transition values, read from the DMA traces, showed a positive shift as compared to the theoretical values calculated by the Fox equation. This suggests a strong H‐bonding interaction between the phenolic resole and PEO as established by Fourier transformed infrared spectroscopy. Flexural test indicated an enhanced flexibility of the blends when compared to the neat phenolic resin. The fracture surface analysis by using a scanning electron microscope (SEM) revealed an increase in plastic deformation with increasing PEO concentration in the blend. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Differential scanning calorimetry analysis of thermoset cure kinetics: Phenolic resole resin

The Differential Scanning Calorimetry (DSC) trace for a commercial phenolic resole resin shows two distinct peaks. Assuming that these represent two independent cure reactions results in a kinetic model of the form: with κ_i = κ_io exp(-B_i/T). The Arrhenius parameters were estimated from a plot of ln(β/T) versus 1/T_p. The parameters, p, n₁, and n₂ were obtained by writing the DSC response predicted by the equation above in terms of a function which contains temperature as the only variable. with $ \theta _i = \left({1/\beta} \right)\int_{T_0}^T {\kappa _i dT \le r_i} $ dT ? r_i and r_i = 1/(1-n_i). Fitting this equation to the DSC response measured at a scan rate of 4°C/min obtains p ≈ 0.66; n₁ ≈ 0.55; n₂ ≈ 2.2; B₁ ≈ 8285; B₂ ≈ 7480; κ₁ ≈ 1. 12 × 10⁸ s^?1; κ₂ ≈ 0.99 × 10⁶ S^?1.     

A new method to predict optimum cure time of rubber compound using dynamic mechanical analysis

The degree of vulcanization of a rubber compound has a big influence on the properties of the final product. Therefore, precisely defining the curing process including optimum cure time is important to ensure the production of final products having high performance. Typically, vulcanization is represented using vulcanization curves. The main types of equipment used for producing vulcanization curves are the oscillating disc rheometer (ODR) and the moving die rheometer (MDR). These can be used to plot graphs of torque versus time at a constant temperature to show how cure is proceeding. Based on the results obtained, optimum cure time (t₉₀) is calculated as the time required for the torque to reach 90% of the maximum achievable torque. In this study, the use of Dynamic Mechanical Analysis (DMA) for assessment of t₉₀ was assessed. DMA was carried out using shear mode isothermal tests to measure the changes in material properties caused by vulcanization. The results revealed that the shear storage modulus (G′), shear loss modulus (G′′) , and tan δ all reflect the vulcanization process, however, tan δ gave the best representation of level of vulcanization. Indeed, the curve of tan δ was able to be used to derive the t₉₀ for rubber compounds and showed good agreement with the results from an MDR. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40008.     

C NMR study on structure, composition and curing behavior of phenol-urea-formaldehyde resole resins

Phenol-urea-formaldehyde (PUF) resole resins were synthesized and analyzed by both liquid and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The liquid ¹³C NMR analysis indicated that the co-condensation reactions between the phenolic ring and the urea unit occurred during the synthesis of the resins. The addition of the urea component effectively reduced the free formaldehyde content in the resin systems. Methylene ether bridges in the resins were found to be mainly associated with the urea units. pH had significant influences on the structure and composition of the resins. Solid-state ¹³C NMR measurements of the cured resins suggested that the pH probably affected the curing mechanism. A longer time and a higher temperature can generally accelerate the curing process and increase the rigidity of the cured network.     

Effect of cure history on dynamic mechanical properties of an epoxy resin

The effect of cure history on the dynamic thermomechanical properties of a high temperature curing epoxy resin has been studied using torsional braid analysis. In isothermal cures “full cure” is not possible except at temperatures above the maximum glass transition temperature (T_g) of the cured resin, hence the necessity of a “post-cure” after lower temperature isothermal cures. The highest T_g and maximum cross-linking in the cured resin was for a linear heating rate of 0.05°C/min from 30 to 200°C; higher heating rates lead to lower glass transition temperatures.